In an electronic ballast for powering gas discharge lamps, it is preferred that the ballast be capable of detecting the presence of functional lamps (i.e., lamps having both filaments intact and being in operational condition) at the ballast output connections. Such detection is useful, for example, in allowing the ballast to provide an appropriate level of heating to the filaments of the lamps, and may also be utilized to provide the ballast with enhanced capabilities for more accurately detecting various types of lamp fault conditions.
A number of existing programmed-start type ballasts utilize a direct current (DC) path through the lamp filaments to provide startup current to a driver circuit for the ballast inverter, thereby ensuring that the inverter will start only if at least one lamp with intact filaments is present at the output connections of the ballast. This approach works well in certain cases, but is often plagued by the problem of excessive power dissipation, especially in those applications for which the starting current requirements of the driver circuit are relatively high; in those cases, the DC path necessarily has a relatively low impedance (to allow higher current flow for meeting the starting current requirements of the driver circuit) which, during steady-state operation of the ballast, results in considerable power dissipation and thus significantly detracts from the overall energy efficiency of the ballast. Accordingly, a need exists for an alternative approach for detecting the presence of functional lamps (i.e., lamps with both filaments intact) that does not entail significant additional power dissipation within the ballast.
Ballasts with driven type inverters usually include some form of protection circuitry for protecting the ballast from excessive power dissipation and/or damage in the event of a lamp fault condition (e.g., removal or failure of one or more lamps). Such protection circuitry typically utilizes certain predetermined voltage thresholds in order to determine whether or not a lamp fault condition is present. In some ballasts, the protection circuitry is designed to accommodate relamping (i.e., replacement of a failed lamp with a new lamp) without requiring that the input power to the ballast be cycled (i.e., the power switch being turned off and then on again) in order to ignite and operate the new lamp. For ballasts that include protection circuitry, it is helpful for the ballast to be able to ascertain, prior to lamp ignition, the presence of lamps with intact filaments connected at the ballast outputs, so as to establish appropriate voltage thresholds for determining whether or not a lamp fault condition is indeed present.
Therefore, a need exists for a ballast that is capable of detecting the presence of lamps with intact filaments in a reliable, cost-effective, and energy-efficient manner. Such a ballast would be capable of providing a number of benefits, including more appropriate levels of filament preheating as well as more accurate detection of lamp fault conditions, and would thus represent a considerable advance over the prior art.